This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems for detecting low velocity flow rates.
Ultrasonic contrast agents enable clinicians to image and quantify structures and functions in the body which are not otherwise readily seen or measured. The ability of ultrasonic contrast agents to resonate nonlinearly when excited with ultrasound can produce a strong harmonic response, enabling clear segmentation of vessels infused with the contrast agent. A sharply defined image can be produced by disrupting the microbubbles of the contrast agent with high MI ultrasound as described in U.S. Pat. No. 5,456,257. The ability to follow the progress of a contrast agent as it infuses the blood vessels of tissue makes it possible to produce measures of myocardial perfusion using contrast, as described in U.S. patent 5,833,613.
Recent studies of lesions such as breast lesions have focused on the vasculature of the lesion. Early detection of breast lesions and definition of the lesion boundaries can often be ascertained by using ultrasound to look for characteristic vascular structures. In addition, changes in lesion growth and development such as those resulting from chemotherapy often manifest themselves at an early point in time by changes in the lesion vasculature. It is anticipated that these studies may be aided by the use of contrast agents. However, the vascular structures involved are tiny, micro-vascular structures with individual vessels conducting minute amounts of blood flow at very low rates of flow. It would be desirable to be able to use contrast enhanced ultrasound to detect the low rates of flow in such micro-vascular structures.
In accordance with the principles of the present invention, ultrasound is used to image micro-vascular structures with the aid of a contrast agent. Slow flow rates are discerned by time interleaving fast time ensembles among different beam directions, with the response to each fast time ensemble used to detect the harmonic response by the pulse inversion technique. The aperiodic timing of an ensemble can be used to smooth undesired nulls in the Doppler spectrum.